bimolecular electron transfer as long as rate
constants are not too far from their diffusion-
controlled limit. The reactivity of the^2 LMCT
state toward both electron donors and acceptors
was studied by monitoring the impact of the
quenchers on both steady-state emission inten-
sity as well as emission lifetime (Fig. 4, A and
B). The methylviologen dication (MV2+)and
arylamines such as diphenylamine (DPA) are
widely used electron-transfer quenching agents
with suitable redox properties. The observed emis-
sion quenching results were attributed to oxidative
and reductive electron transfer, respectively, gen-
erating [FeIV(phtmeimb) 2 ]2+and MV•+(change
in Gibbs free energyDGo=−1.08 eV) in the former
case and [FeII(phtmeimb) 2 ] and DPA•+(DGo=
−0.55 eV) in the latter.
Bimolecular quenching rate constants in ace-
tonitrile for dynamic quenching were determined
from Stern-Volmer plots of emission lifetimest 0 /t
(Fig. 4C); rates were diffusion controlled for DPA
(kq=1.4×10^10 M−^1 s−^1 ) and only somewhat
lower even for MV2+(kq= 2.7 × 10^9 M−^1 s−^1 ).
Although no indications of ground-state com-
plexation were observed with MV2+,additional
static quenching by DPA was evident from the
curved Stern-Volmer plot of steady-state inten-
sitiesI 0 /I.Withbothquenchers,theformationof
electron transfer products was unambiguously
confirmed by transient absorption spectroscopy
(Fig.4D).SpectraafterquenchingbyDPAshow
characteristic absorption of the donor cation
radical peaking at 680 nm ( 29 ) and of the FeII
state rising toward the ultraviolet region (<420
nm) (see also fig. S18). Also quenching by MV2+
resulted in transient absorption spectra that
display the well-known absorption features of
the acceptor radical MV•+(at 396 and 606 nm)
( 30 ) together with the broad 700-nm band of
the Fe(IV) complex (see fig. S18). In the flash
photolysis experiments, the excited state at an
initial concentration of ~1.5 × 10^5 M(determined
by actinometry with [Ru(bpy) 3 ]2+) was quenched
with efficiencies of ~0.7 by 0.25 M MV2+and
nearlyunityby0.2MDPA.Fromtheinitialcon-
centrations of MV•+(De 396 =41,800M−^1 cm−^1 )
( 30 )andDPA•+(De 680 =19,200M−^1 cm−^1 )( 29 ),
we estimate that in both quenching reactionsabout 5% of the charge-separated products escape
geminate recombination in the solvent cage. The
diffusional recombinationoftheseparatedpro-
ducts occurs on the time scale of 100msand,in
case of DPA, proceeds via oxidation of the FeIII
ground state by the donor radical (see supple-
mentary materials). Although the cage escape
yields values three to five times lower than those
typically observed in the quenching of the^3 MLCT
state of [Ru(bpy) 3 ]2+, that latter process benefits
from spin restrictions to back electron transfer in
the triplet radical pair ( 31 );theyieldscompare
very favorably to the negligible cage escape en-
countered in other cases of spin-allowed back
electron transfer in the quenching of, for in-
stance, singlet excitedstates of porphyrins ( 32 ).
The extended CT lifetimes in [Fe(phtmeimb) 2 ]+
were accomplished without substantial loss of the
>2-eV excited-state energy, providing the^2 LMCT
state with a superior combination of oxidative and
reductive power exceeding the corresponding values
of the archetypal [Ru(bpy) 3 ]2+sensitizer (fig. S26).
Thermodynamically, the^2 LMCT state should be
capable of oxidizing or reducing a wide range of
molecular donors and acceptors and p- or n-type
semiconductor materialsand of driving demand-
ing photocatalytic reactions such as water oxi-
dation or carbon dioxide reduction, which could
further benefit from the complex’s intrinsic
stability (fig. S26).
The 2% PL quantum yield also raises the
prospect of applying Fe-NHC systems to bio-
sensors and organic light-emitting diodes ( 33 ).
These applications would benefit from the in-
trinsic low toxicity and earth abundance of Fe
complexes, as well as the insensitivity of the(^2) LMCT excited state of [Fe(phtmeimb)
2 ]
+to oxygen.
Moreover, because both the ground and LMCT
excited states of the FeIIIlight-emitting complex
are doublets, they will not suffer from the endemic
singlet-versus-triplet formation problem ( 33 )
of typical rare-earth light emitting complexes.
Taken together, our results suggest that the
(^2) LMCT state deserves more attention as a photo-
functional state for iron and other transition
metals as well.
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Fig. 4. Reactivity of the^2 LMCT excited state of [Fe(phtmeimb) 2 ]+toward electron donors
and acceptors.Emission quenching was monitored by emission lifetime (TCSPC traces with
exponential fits) and steady-state emission spectra (insets) for increasing concentrations
(black to cyan) of (A) diphenylamine donor (0, 0.005, 0.01, 0.02, 0.05, 0.1, and 0.2 M) and
(B) methylviologen acceptor (0, 0.01, 0.025, 0.05, 0.1, 0.25, and 0.5 M) in acetonitrile.
(C) Stern-Volmer plots for steady-state intensity (open symbols) and lifetime data (solid symbols)
from quenching experiments with diphenylamine (triangles) and methylviologen (circles).
(D) Transient absorption spectra after laser flash excitation (465 nm) of [Fe(phtmeimb) 2 ]+,
monitoring products of oxidative quenching by methylviologen (0.25 M) 500 ns after excitation
(black) and of reductive quenching by diphenylamine (0.2 M) 100 ns after excitation (red).
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